Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants may be composed of gaseous compounds such as, for example, the oxides of nitrogen (NOx). Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of NOx emitted from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. In order to ensure compliance with the regulation of these compounds, some engine manufacturers have implemented a strategy called Selective Catalytic Reduction (SCR).
SCR is a process where gaseous or liquid reductant (most commonly urea) is added to the exhaust gas stream of an engine and is absorbed onto a catalyst. The reductant reacts with NOx in the exhaust gas to form H2O and N2. Although SCR can be effective, it is most effective when a concentration of NO to NO2 supplied to the SCR is about 1:1. In order to achieve this optimum ratio, a Diesel Oxidation Catalyst (DOC) is often located upstream of the SCR to convert NO to NO2.
In addition to facilitating the reduction process of the SCR, the NO2 produced (i.e., converted from NO) by the DOC can also facilitate the combustion of collected particulate matter. Specifically, a particulate trap is commonly used to collect unburned particulates also known as soot. Over time, the particulate matter builds up in the trap and, if left unchecked, the particulate trap could negatively affect performance of the engine. As such, the particulate matter collected by the trap must be periodically removed through a process called regeneration. To regenerate the particulate trap, a liquid catalyst (typically diesel fuel) is injected into the exhaust flow upstream of the trap. The fuel, in the presence of NO2, ignites and burns away the collected particulate matter. An efficiency of the regeneration process, like the SCR process, can be affected by the amount of NO2 present in the exhaust flow.
It is known that the ratio of NO to NO2 contained in the exhaust stream exiting the DOC may vary based at least partially on the flow rate of exhaust passing through the DOC and on a temperature of the exhaust. In the past, the flow rate of exhaust passing through the DOC has been almost completely dependent on operation of the engine (i.e., on a flow rate of gases combusted and subsequently exhausted from the engine). Thus, the conversion rate of NO to NO2 has been controlled by varying a temperature of the exhaust.
A system implementing such a strategy is described in U.S. Pat. No. 6,807,807 (the '807 patent) issued to Kagenishi on Oct. 26, 2004. The '807 patent discloses an exhaust gas purifying apparatus having a particulate filter, an oxidation catalyst, a front oxidation catalyst, a bypass path, and a passage switching device disposed in an exhaust path. The front oxidation catalyst is disposed further upstream than the oxidation catalyst. The bypass path bypasses the upstream side and the downstream side of the front oxidation catalyst during normal operation such that the entire gas flow passes only through the oxidation catalyst and the particulate trap. The passage switching device switches the flow of exhaust gas to the front oxidation catalyst from the bypass path during filter regeneration. That is, when the filter is forcibly recovered (i.e., regenerated), the temperature of the exhaust gas is raised by an injection of fuel and is made to pass through the front oxidation catalyst such that the oxidation catalyst is sufficiently warmed and an adequate amount of NO is converted to NO2 for optimum regeneration of the filter. After the oxidation catalyst is properly activated, the flow of exhaust gas is switched back to pass through the bypass path, and regeneration of the filter is completed in the presence of NO2.
Although somewhat effective at controlling the conversion of NO to NO2 during filter regeneration, the exhaust gas purifying apparatus of the '807 patent may be complex, costly, and lack applicability. That is, the apparatus of the '807 patent requires multiple oxidation catalysts and complicated bypass and heating structures. These components increase the complexity of the system, as well as part and assembly cost. In addition, the fuel used to heat the exhaust reduces an efficiency of the engine. Further, during some situations, such as at startup or during operation in cold ambient conditions, the elevated temperature of the exhaust alone may be insufficient to properly activate the catalyst. And, temperature control of the catalyst for use in regenerating a particulate trap may be suboptimal when used with an SCR device.
The system of the present disclosure solves one or more of the problems set forth above.